1. Field of the Invention
The invention relates to an implantable shock electrode line for treating a biological tissue and/or an organ with electroshock therapy and an implantable shock electrode arrangement.
2. Description of the Related Art
Implantable electrode lines, in particular cardiac electrode lines for endocardial sensing of cardiac action potentials and/or for electric stimulation or defibrillation of the heart are already known in a wide variety and have long been in practical use on a large scale in combination with implanted cardiac pacemakers or defibrillators. Endocardial systems (ICDs) dramatically reduce morbidity and mortality and make the therapeutic process a realistic option for large groups of patients.
FIG. 1 shows the basic design of an implantable defibrillation arrangement 1, such as that known from U.S. Pat. No. 5,531,766. To stimulate the heart H of a patient P, a defibrillator 3 is electrically connected to an electrode line 5 having at least one electrode (“shock electrode”) 7, which is located at its distal end and is placed in the patient's heart. As a rule, defibrillation is performed via a current path between this electrode and a counter electrode located a greater or lesser distance away from the heart to be stimulated. The position and structure of the electrode play an important role in efficient stimulation and/or defibrillation. The housing 8 of the defibrillator 3 containing the units for detection of heart signals and for generating electric pulses may act as counter electrodes. Alternatively, a counter electrode may also be mounted on the outside of the housing.
Like a pacemaker implantation, the ICD method involves access via the cephalic vein or the subclavian vein and positioning of a defibrillation probe in the right ventricle. ICD probes having a single shock electrode for placement in the right ventricle are available or have a second electrode positioned further proximally, typically in the right atrium, in the superior vena cava or in the subclavian vein.
In addition, “hot can” systems offer two possible configurations: a “single-coil” probe with the shock vector from the right ventricle to the ICD housing (referred to as unipolar) and a “dual coil” probe with a defibrillation field between the right ventricle, the superior vena cava and the housing (“triad” configuration).
U.S. Pat. No. 5,203,348 describes a defibrillation electrode for subcutaneous administration in the form of a spiral design with a large area, forming the distal end of the feeder line of the defibrillation electrode. The coiled shape is used here to increase the effective electrode surface area.
The coil-like shock electrodes are coiled from one or more parallel wires, strips or small coils. The electric resistance here is constant over the length of the wire and/or strip.
The current density distribution of a coil-like electrode depends on the electrically active surface area, the electric resistance of the coil material and, apart from the body structures of the patient, the location of the electrodes relative to one another, so the current density distribution can be influenced only to a limited extent with traditional designs. Furthermore, an electric shock administered to the heart or chest does not produce a perfectly uniform electric field. In a so-called “dual-coil system,” the shock coils are arranged in succession, for example, rather than being axially parallel, so that the tissue and/or organs between the electrodes is/are exposed to a heterogeneous field distribution. In regions of a higher field strength (typically close to the defibrillation electrode), the myocardium has greater effects in depolarization and repolarization than cells in the weaker electric field. This known variability of the cellular response in a heterogeneous shock field is known as a “graded response.”
The defibrillation threshold (DFT) is an important factor that must be taken into account in defibrillation therapy. The defibrillation threshold corresponds to the lowest shock energy with which ventricular fibrillation is in fact terminated. The defibrillation threshold depends on the cardiac cells reached “last,” so it is possible to assume that when there is an uneven field distribution, certain tissue zones are supplied with too much energy in an unnecessary manner. With a balanced field distribution, however, the DFT can be lowered.
The electric field can be influenced by changing the polarity of the shock electrodes as disclosed in U.S. Pat. No. 6,449,506. However, this makes it possible to adjust the transition impedance to the electrolyte only within narrow limits. The transition impedance to the electrolyte is understood to refer to the total resistance of the current path between the shock electrode and the counter electrode through an electrolytic material (tissue, organ, blood).